In the field of LEDs, vigorous researches are recently conducted to use white LEDs for lighting, in accordance with development of white LEDs with higher luminance. Having a feature of a point light source, LEDs are expected to replace halogen lamps and the like which have been conventionally used as spotlighting in a shop, a museum, a showroom, and the like.
At present, a mainstream white LED includes a combination of an LED bare chip emitting blue light and a phosphor that is excited by the blue light to emit yellow light. The blue light and the yellow light mix together, to produce white light. Generally, to form this white LED, the bare chip is first mounted on a lead frame, printed wiring board, or the like. After this, a resin containing particles of a phosphor is dropped onto the bare chip, which forms the phosphor on and around the bare chip (see Japanese patent No. 2998696).
Here, although such a current white LED has higher luminance than previous ones, a single LED bare chip can only produce a small amount of light. Therefore, multiple LED bare chips are mounted on a printed wiring board, so as to produce a sufficient amount of light for a lighting apparatus. Furthermore, to make effective use of light emitted by each LED bare chip, a printed wiring board to which a reflective board is attached (hereinafter referred to as a printed wiring board with a reflective board), or a printed wiring board which includes a reflective film formed in an insulating layer (hereinafter referred to as a printed wiring board with a reflective film) is used as a mounting substrate.
The reflective board is an aluminum board or the like in which a taper hole is provided in correspondence with a location, on the printed wiring board, at which each LED bare chip is to be mounted. The reflective board is adhered to the printed wiring board using an adhesive agent. On the other hand, the printed wiring board with a reflective film is composed of two insulating layers, for example. In the upper insulating layer, a taper hole is provided, and an aluminum reflective film is formed on a wall of the taper hole. In this case, each LED bare chip is mounted in a location, on the lower insulating layer, which corresponds to a center of the taper hole. The taper hole provided in the reflective board and the taper hole with the aluminum reflective film on its wall provided in the upper insulating layer are hereinafter collectively referred to as a reflective hole.
If an LED bare chip is mounted on either of the two different printed wiring boards described above, light emitted from a side surface of the LED bare chip is reflected forward by the wall of the reflective hole. Thus, the light from the side surface is emitted in the same direction as light from a front surface of the LED bare chip. Thus, light emitted from the LED bare chip can be efficiently used. Here, it is desirable to make a distance between the LED bare chip and the wall of the reflective hole as small as possible, in order to minimize a size of a lighting apparatus and the like and maximize a light collection efficiency. In other words, it is preferable to make a diameter of the reflective hole as small as possible.
Here, when a resin containing particles of a phosphor is dropped onto the LED bare chip, which has been mounted on one of the above printed wiring boards, in a mounting process of a white LED, the resin fills the reflective hole. Which is to say, a phosphor is formed so as to be in contact with the wall of the reflective hole. This means that the wall of the reflective hole does not appropriately reflect light emitted from a side surface of the LED bare chip forward. Accordingly, a desired light collection efficiency can not be achieved.
The U.S. Pat. No. 6,650,044 discloses a technique to cover a bare chip with a phosphor in a different method from a method in which a resin containing particles of a phosphor is dropped onto a bare chip. This technique is described in the following with reference to FIGS. 1 and 2.
As shown in FIG. 1, a plurality of bare chips 900 (for example, six bare chips 900A to 900F) are flip-chip mounted on a printed wiring board 902. After this, a stencil 904 is overlaid on the printed wiring board 902 in the following manner. Here, the stencil 904 is a stainless-steel plate which has a slightly larger thickness than the bare chips 900. In the stencil 904, through holes 906A to 906F that are slightly larger than the bare chips 900 in size are provided so as to correspond to the bare chips 900. The stencil 904 is aligned with the printed wiring board 902 so that the bare chips 900A to 900F are respectively to be fitted into the through holes 906A to 906F, and then overlaid.
FIG. 2A illustrates a cross-section of the printed wiring board 902 on which the stencil 904 is overlaid. After this, the through holes 906 are filled with a material 908 containing particles of a phosphor (shown in FIG. 2B). Subsequently, the stencil 904 is taken away (shown in FIG. 2C), and the material 908 is then cured. In this way, a white LED including a phosphor (908) can be mounted on the printed wiring board 902. Furthermore, the phosphor (908) is deposited on and around each of the bare chips 900 at a substantially even thickness.
This technique is applicable to mount a white LED on a printed wiring board with a reflective board in such a manner that a white LED is first mounted on the printed wiring board, and a reflective board is then adhered to the printed wiring board. However, the technique is difficult to be used for mounting a white LED on a printed wiring board with a reflective film.
Here, a printed wiring board with a reflective board is not as preferable as a printed wiring board with a reflective film for the following reasons. Firstly, it is difficult to form an adhesive layer adhering a reflective board to a printed wiring board, at an even thickness. Therefore, a relative location of each reflective hole provided in the reflective board with respect to a corresponding white LED in a direction of light emission is not uniform. Accordingly, a light collection efficiency of each reflective hole is not uniform. Secondly, a predetermined relative location between a reflective hole and a bare chip (a white LED chip) is not likely to be achieved. This is because a bare chip is not precisely mounted on a designed location on a printed wiring board, and a reflective board can not be perfectly aligned with the printed wiring board. This degrades a light collection efficiency. Thirdly, when a printed wiring board with a reflective board is utilized, an additional step is required for adhering the reflective board which has been separately manufactured to the printed wiring board. These problems are not created if a printed wiring board with a reflective film, which has a reflective hole formed as part of the printed wiring board, is utilized.
Accordingly, a printed wiring board with a reflective film is more suitable than a printed wiring board with a reflective board, as a constituent of a lighting module or the like including a white LED.
It should be noted that the above-mentioned problems are not particular to a white LED, but common to any semiconductor light emitting devices which include a combination of an LED bare chip and a phosphor to produce light of a desired color.
In light of the above-described problems, an object of the present invention is to provide a semiconductor light emitting device that can be mounted on a printed wiring board with a reflective film in such a manner that a phosphor of the light emitting device is not in contact with a wall of a reflective hole in the printed wiring board, a manufacturing method for the same, and a lighting module, a lighting apparatus and a display element including the semiconductor light emitting device.